I added cheap plastic Fiamm air horns years ago, and wired them through a relay to the stock horn wire. A very short beep uses the stock horns without giving the air pump time to spin up. Anything longer and the much louder air horns let you know they're there. Of course, seems I only use the horn during annual inspections. For some years, the air pump had rusted and stopped; but I finally dismantled it and clean and lubed it, and it's been OK since.
In a protected space, the Fiamm electric replacement horn will work well. Hmmm, I seem to recall installing one or a pair of these on my motorbike long ago. The horn was much more authoritative than the wimpy 'hmeep, hmeep' produced by the standard Suksuzi horn. Alas, the exhaust pipe melted it after a while.